The wireless local area network (WLAN) is a mobile network surfing technique that has the features of high mobility to free from the constraint of the wired network and easy installation. Hence these days, the WLAN can not only meet network requirements in houses, it is also increasingly popular in public locations such as airports, restaurants, cafes and the like to provide network surfing services for users whenever needed.
In order to enable the technique of WLAN to be widely adopted, industry standards or protocols must be established to ensure that equipment produced by all vendors is compatible and has the required reliability. Those standards have been established by The Institute of Electrical and Electronic Engineers (IEEE). The earliest specification is IEEE 802.11, announced in 1997. Then IEEE 802.11a and IEEE 802.11b were announced in September 1999.
The earlier specification defines applications for a radio frequency spectrum 2.4 GHz, and provides transmission speed specifications for 1 Mbps and 2 Mbps and many basic signal transmission methods and services. Nowadays, IEEE 802.11a and IEEE 802.11b have gradually become the main-stream standards, using respectively two frequency spectrums of 5.8 GHz and 2.4 GHz.
IEEE 802.11a and IEEE 802.11b are two incompatible frequency spectrums. Some vendors have endeavored in technical developments for both spectrums. In addition, the U.S.A, Europe and Japan do not permit 5 GHz to be used outdoors in order to prevent interference with satellite bound mobile phones and weather radars. Every country has restrictions on emission power of high frequency communication devices. This is especially true for the 5 GHz frequency spectrum. Depending on different regulations of various countries, allocation of this spectrum also is different. In such circumstances, it is necessary to enable 2.4 GHz to coexist with 5 GHz.
With the 2.4 GHz frequency spectrum and the 5 GHz frequency spectrum coexist on the same communication chip, the antenna must also be able to simultaneously receive the frequencies of these two frequency spectrums. Thus double frequency antenna has become a mandatory element for WLAN implementation.